libstdc++: Improve unordered containers == operator (PR 91263)

libstdc++: Improve unordered containers == operator (PR 91263)

Avoid comparing elements with operator== multiple times by replacing
uses of find and equal_range with equivalent inlined code that uses
operator== instead of the container's equality comparison predicate.
This is valid because the standard requires that operator== is a
refinement of the equality predicate.

Also replace the _S_is_permutation function with std::is_permutation,
which wasn't yet implemented when this code was first written.

	PR libstdc++/91263
	* include/bits/hashtable.h (_Hashtable<>): Make _Equality<> friend.
	* include/bits/hashtable_policy.h: Include <bits/stl_algo.h>.
	(_Equality_base): Remove.
	(_Equality<>::_M_equal): Review implementation. Use
	std::is_permutation.
	* testsuite/23_containers/unordered_multiset/operators/1.cc
	(Hash, Equal, test02, test03): New.
	* testsuite/23_containers/unordered_set/operators/1.cc
	(Hash, Equal, test02, test03): New.